Teleprinter system



DCC. 29, G. K|NG TELEPRINTER SYSTEM 2 SheetshSheet 2 Filed Oct. 11. 1956 .tm 39@ Si INVENToR.

A T0 EY GEORGE L. K/NG l.. r I l II, l I l l |..Ml,l/|, l l I l l l.

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`representing a Space function.

TELEPRINTER SYSTEM George L. King, Morris Plains,

Frequency Laboratories, of New Jersey Application ctober 11, 1956, Serial No. 615,376

11 Claims. (Cl. 178-2) NJ., assignor to Radio Boonton, NJ., a corporation This invention relates to a teleprinter system and more particularly to a system of this type incorporating a novel D.C. to A.C. keying arrangement whereby remotelyspaced printers may be connected together by a Z-Wire A.C. line and each printer may be used for both transmitting and receiving functions.

In teleprinter systems of the type to which this inven- ,as a Space signal. The coded Mark and Space signals are distinguished at the receiving printer, so that only the Mark signals are effective to cause operation of the selector magnet mechanism and bring about the power operation of the type bar corresponding to the particular received code character.

Normally, in a simple teleprinter system involving two printers, the selector magnet of each printer is connected in one common D.C. loop having a D.C. voltage supply. Each printer contains a keyboard which operates a set of keying contacts which are in series with the common D.C. loop. Therefore, manual operation of the keyhoard of either printer will cause identical information to he printed on both local and remote machines. It also follows that manual operation of the keyboard of the receiving printer will break up the local copy being made on the sending printer. This break feature is desirable since it can be used to signify that the sending operator should cease typing and stand by to receive. In such an arrangement there are practical limits which prevent very long D.C. loop connections between the printers.

A system made in accordance with this invention permits the use of a nominal 600 ohm, 2-wire line instead of the D.C. loop. It also permits the use of a radio trans- Pate-nt O mission link between the two printers. This system also i retains the operational features of the printers as mentioned in the preceding paragraph. Both transmitting and .receiving operations are possible with either printer, each function being controlled at the printer keyboard and without requiring any manual adjustments of the equipments for either transmitting or receiving. Frequency shift audio signals are employed on the 2-wire line, one carrierchannel being used for each direction of communication. teleprinter selector magnet, when the keyboard, is operated, are employed to shift the frequency of an audio frequency carrier above and below the center frequency of the carrier. Thus, two discrete audio frequencies are generated, one representing a Mark function and one At the receiving teleprinter the Mark and Space tones are converted to D.C.

' pulses of current for operating the selector magnet.. Each .equipment includes a tone oscillator for transmitting 1n- The Mark and Space functions of theV 2,919,301 APaty tented Dec. 29, 1959 ICC . 2 formation and a signal amplifier, discriminator and keying circuit for receiving information. Different transmitting tones are transmitted in each direction with the input of each equipment filtered to accept only the tones originating at the remote transmitting printer andk to reject tones of its own local transmitting oscillator. The D.C. loop circuit for the selector magnet of each individual printer includes an electron keying tube, the keyboard keying contacts and a `D.C. voltagersupply. When receiving, the switching action of the keying tube is controlled by received signals from the remote printerinstallation, such signals being rectified in a discriminator circuit and applied to the tube grid. A Mark signal causes a positive D.C. potential to be applied to the grid of the keying tube whereas a Space signal is converted to a negative bias on the grid. The result is neutral keying, current flowing through the selector magnet of the printer during a Mark signal only. When transmitting, the selec-k tor magnet loop is opened and closed through the keying contacts and the kelectron keying tube is biased so that it appears as a low value of resistance in series with the loop. A transmitting tone oscillator circuit is connected to the loop circuit. The arrangement of the circuit is such that the D.f`. loop voltage is applied tothe input of the oscillator circuit' during a Mark (closed keying contact) and removed during a Space (open keying contact). The presence and absence of the loop Voltage at the input to the oscillator circuit is employed to shift the oscillator frequency to Mark and Space audio tones respectively.

The electron keying tube in series with the printer loop circuit must act to close the circuit while the printer keyboard is being operated for transmitting information. This requires a steady Mark signall from the equipment at the remote installation which is receiving the Mark and Space signals. This is accomplished by having the oscillator of the receiving printer transmit an uninterrupted Mark signal, which is possible in my circuit because the complete absence of printer loop voltage (during a space signal) is required to shift the oscillator, the latter occurring only when the loop circuit is switched by the printer keyboard contacts. lt is obvious that operation of the keyboard of the receiving printer will not only break up the copy being printed but will Valso interrupt the steady Mark signal which is being transmitted to the sending printer, thus breaking up its local copy. This operational feature is the same as that obtained between two printers having their selector magnets in one common D.C. loop circuit.

My system also includes a Mark-Hold circuit in order to close .the selector magnet loop for transmitting purposes, in the event that the equipment failed to receive a steady Mark signal from the remote printer.

The electron keying tube circuit is normally biased for zero selector magnet current until areceived Mark signal overcomes this bias and causes current to ow, thus closing the selector magnet D.C. loop.V The function 0f the Space signal is to insure complete cut-ofi of the keying tube, effectively opening the selector magnet circuit. Therefore, during the absence of either Mark or Space signals, the selector magnet would be in a Space condition (open circuited). Thus, complete' absence of signals simulates a steady Space signal. The Mark-Hold circuit is arranged to place the selector magnet loop in a closed-circuit condition only when ythe absence of received tones exists for a time period greater than the Space signal time encountered in the normal code for` intelligence. This prevents the operation of the Mark-Hold circuit from interfering with the normal receiving function of the system and at the same time permits signal transmission in one direction during failure of transmission in the opposite direction.

An object ofk this invention is the provision of a novel teleprinter system permitting the use of a two-wire A.C. line instead of the normally employed D.C. loop between two remote printers.

An object of this invention is the provision of a teleprinter system wherein two remotely-disposed printers are connected together by a two-wire A.C. line and wherein each printer can be used for both transmitting and receiving.

An object of this invention is the provision of a teleprinter system comprising two equipments each including a tone oscillator for transmitting purposes, and a signal amplifier, discriminator and keying circuit for receiving purposes, the said equipments adapted for connection to a two-wire line and the tone oscillator of one equipment generating tones of a character diiterent from those of the other equipment, and the receiving circuits of each equipment designed to accept the tones from the remote equipment only.

An object of this invention is the provision of a system comprised of two teleprinter keying equipments working in conjunction with each other but remotely located, means whereby each equipment is capable of keying a teleprinter when said equipment is energized by interrupted signals known as Mark and Space signals originating at the remote teleprinter and keying equipment, such signals being a function of operation of the remotely located printer keyboard contacts, and means whereby the ability of the keyboard keying contacts of either printer to be operated so as to produce correct local copy depends on said printer and its associated keying equipment being controlled by an uninterrupted Mark signal from the remote equipment, the uninterrupted Mark signal being a function of the remote printer keyboard contacts being inoperative.

An object of this invention is the provision of a teleprinter keying apparatus comprising a keying tube having its cathode and anode connected in a series circuit that includes a D.C. voltage source and printer type bar contacts, means to apply a succession of positive and negative pulses conforming to coded intelligence originating from a remotely-disposed similar teleprinter keying apparatus to the grid of the tube thereby causing the tube to alternately close and open the series circuit, means to apply a steady positive signal voltage originating at the remote apparatus to the grid of the tube causing it to conduct steadily so that the series circuit can be opened and closed by means of the type bar contacts, circuit elements to apply a steady voltage to said grid when positive or negative voltages as fuuctions of the operation of the remote apparatus are not applied to the tube grid, and means of disconnecting this circuit from the grid when the first positive signal voltage from the remote apparatus is received.

These and other objects and advantages will become apparent from the following description when taken with the accompanying drawings illustrating the invention. It will be understood, however, that the drawings are for purposes of illustration and are not to be construed as defining the scope or limits of the invention, reference being had for the latter purposes to the claims appended hereto.

In the drawings wherein like reference characters denote like parts in the several views:

Figure l is a block diagram of a single teleprinter apparatus made in accordance with this invention;

Figure 2, is a fragmentary circuit diagram of the last two stages of the receiving channel amplier and the mark-hold circuit.

Reference is now made to Figure l which is a block diagram of a single teleprinter equipment having a pair of input-output terminals 10. It may here be pointed out that a similar teleprinter equipment, disposed at a remote location and also provided with input-output terminals, is connected to the Figure l equipment by a nominal 600 ohm, two-wire, A.C. line, to form a teleprinter system. The two pieces of equipment are identical except that different transmission tones are transmitted in each direction across the two wire line. Specifically, if the A.C. current pulses transmitted by the Figure 1 equipment have a frequency of 180 cycles per second the remote equipment will be tuned to receive such pulses. Under these conditions, the remote equipment will transmit pulses having a frequency of 220 cycles per second and the Figure 1 equipment will be tuned to receive such pulses.

As shown in Figure 1, the electronic components are arranged in two channels, namely, a transmitting channel and a receiving channel each connected on one side to the input-output terminals 10 and on the other side to appropriate electrodes of a keying tube 11 which may be a type 6W6. The cathode 12 and anode 13 of the keying tube are connected in a series circuit that includes a D.C. voltage source, such as the battery 14, the printer selector magnet 15 and the key contacts 16. The actual printer is of conventional construction with the key contacts normally closed, that is, the key contacts close the circuit Ibetween the battery 14 and the selector magnet until a key is depressed as in typing or until such key is operated electromagnetically when the printer is operated electronically, as in receiving transmitted intelligence.

The grid 17, of the keying tube, normally is biased with a positive voltage so that the tube normally is conducting, as will be described in detail hereinbelow. Consequently, the series circuit consisting of the tube 11, selector magnet 15, key contacts 16 and battery 14 normally is closed. The receiving lilter 1S is tuned to pass one carrier frequency with its corresponding Mark and Space signals and attenuates adjacent carrier channels by more than 40 db. In the example under discussion, the lter 18 will pass the 220 cycle pulses transmitted by the remote equipment. On the other hand, the trans mitting ilter 38 is tuned to pass only the transmitting frequency of the particular equipment, in this case, cycles per second. Those signals which pass through the receiving filter 18 are applied to a limiting amplifier 20 which is of conventional construction utilizing three resistance-capacitance coupled stages having both grid and plate limiting characteristics. To permit the amplilier to operate at the greatest possible telegraph transmission speed, while maintaining a minimum diierence of level between marking and spacing carrier frequencies, the time constants of the R-C coupling networks are no greater than the period of the lowest carrier frequency employed. The limited signals are then applied to a tuned discriminator 21, of conventional design, which converts the Mark signals to positive D.C. voltages and the Space signals to negative D.C. voltages. These D.C. signals are passed through two stages of amplitication, namely the amplifiers 22 and 23. Since two stages of amplification are employed at this point, the resulting square waves applied to the grid 17 of the keying tube 11 will be in-phase with the output signals of the discriminator.

The neon tube 24 couples the plate of the amplifier 22 to the grid of the amplifier 23. The negative voltage output of the discrirninator (corresponding to a Space signal) is sufficient to cut oi plate current ow in the ampliiier 22 and under this condition the plate voltage will be of a high enough level to cause the neon tube 24 to ignite. When a positive voltage (corresponding to a Mark signal) appears at the discriminator output the magnitude of such positive voltage is suicient to exceed the normal negative grid bias on the amplifier stage 22! allows a negative voltage to be applied to the grid of l ,amplifier 23 causing plate current cut-ofi` and a rise in plate potential. When neon tube 24 ignites the grid of amplifier 23 is driven positive and plate current saturation with a corresponding drop in plate potential occurs. The neon tube 25 couples the plate of the amplifier 23 to the grid 17 of the keying tube 11. The operation of the amplifier 23 is similarto amplifier 22 and the reversal of polarity of the output voltage to that of the voltage `applied to its grid will result in a positive Voltage being applied to the keying tube grid 17 when a Mark signal is received. The saturation plate current of keying tube 11 then fiows through the selector magnet 15. When a Space signal is received neon tube 25 extinguishes and the negative grid voltage on keying tube 11 causes plate current cut-off.

Thus, when a sequence of spaced wave pulses corresponding to a specific character is received at the equipment terminals 10, correspondingly-spaced plus and minus voltages will `be applied to the grid of the keying tube. The positive voltage pulses cause the keying tube to be conductive thereby closing the circuit through the printer selector magnet and the negative voltage pulses block the keying tube thereby opening the selector magnet circuit. The precise time sequence and the order of Mark and Space signals forming the transmitted character results in a selection of a corresponding selector magnet whereby operation of the power driven printer type bar causes the character to be printed on the receiving typewriter. At the input to amplifier 22 the transition from Mark to Space signals and from Space to Mark signals, in any code sequence of pulses, will produce a sloping voltage curve. However, the neon tubes 24 and 25 have discrete firing voltage points which will cause the amplifier to have an output of square wave form. Such square Waves will be of constant amplitude due to the voltageregulating characteristics of the neon tubes and the volt- .age regulation employed in both the plate voltage supply and the negative bias voltage supply. With neon tube coupling and the magnitude of the grid voltages employed in the amplifiers, variations in vacuum tube characteristics will introduce a minimum of telegraph keying bias.

When no signals are received at the equipment terminals 10, the fixed bias at the input of amplifier 22 will cause a steady spacing voltage of -32 volts to be applied to the grid of the keying tube 11. Since, for transmitting purposes the printer circuit must normally remain closed, I provide a mark-hold circuit for this purpose. The mark-hold circuit is a. two stage amplifier, the one stage 26 being driven by the output voltage of the receiving channel amplifier 22 and the other stage 27 having its output connected `to the grid of the keying tube 11 through a switch 28 and a neon tube 29. Interposed between the amplifier stages 26 and 27 is a neon tube 30 and a timedelay circuit 31. Three stages of amplification exist between the discriminator output and the Akeying tube through the mark-hold circuit in contrast to two stages of normal signal amplification through the amplifiers 22 and 23. Therefore, the polarities of the output voltages; from the mark-hold circuit, for received Mark and Space signals, will be opposite to those applied to the keying tube through the amplifier 23. It will be seen that the mark-hold circuit is connected in parallel with the `amplifier 23 and the neon tube 25. Thus, when a Space signal is received at the equipment terminals the corresponding positive voltage pulse at the output of kamplifier 22 passes through a double phase reversal in the markhold circuit whereby a positive voltage is applied to the grid of the keying tube. The appearance of such positive voltage on the keying tube grid closes the series circuit of the printer in response to the received Space signal as Well as under conditions when no signal is being received. To prevent the operation of the mark-hold circuit from locking in the printer circuit during the normal Vvreceiving function of the equipment, the time delay circuit 31 is interposed in the grid circuit of the mark-hold amplifier stage 27. As will be described in more detail hereinbelow, the time delay circuit includes an lR-C circuit having a time constant sufcient to insure that during the normal spacing time in one code character the positive voltage from the mark-hold circuit cannot be applied to the grid of the keying tube. However, if the spacing time exceeds that which normally is encountered in a code character, or if continuous spacing-voltage is applied to the equipment, the keying tube is rendered conducting.

VRelease of the keying circuitfrom such mark-hold condition occurs at the moment the first Mark signal is received by the equipment. ASwitch 28 lpermits disconnecting the Mark-Hold circuit from the grid of tube 11 when its operation is not desired. One side of neon tube 29 is then grounded. In either case, since neon tube 29 is ignited when no signals are being received it can be used as an indicating device for signal failure.

The keying circuit for the printer is designed for neutral operation with the keying tube operating as an on-of switch in response to received Mark and `Space signals representing coded characters. Keying is accomplished by positive marking voltages, passing through the equipment receiving channel, which `produce saturating Aplate current in the printer loop, and negative spacing voltages which bias the grid for plate cut-off in the'loop. In order to transmit coded characters and also to print local cop-y the keying tube must maintain the printer loop circuit closed either by a continuous positive marking voltage applied to the keying tube grid :from the receiving channel amplifiers or from the Mark-Hold circuit. This requirement makes it possible to have the received signal break the local copy by a series of negative spacing voltages.

With the'keying tube in continuous current conducting condition when no signals are being received,`transmis sion in one direction in a system is possible if transmission failure occurs in the opposite direction. The transmitting channel comprises the amplifier 33, a neon tube 34, an oscillator keyer 35, a frequency-shifty oscillator 36, a buffer amplifier 37 and the transmitting filter 38. The input signals to the amplifier 33 are the loop voltages -in respect to ground which appear across the voltage divider 39. When keying contacts are operated a positive voltage occurs during marking current fiow in the loop and zero voltage during spacing or no current fiow. The grid circuit of vamplifier 33 is biased by a negative voltage. A positive signal voltage from the selector magnet loop circuit will overcome this bias and drive the grid of amplifier 33 to Zero bias and cause a drop in plate potential. A zero voltage signal from the loop will permit the negative bias on the grid of amplifier 33 to cause a lise in plate potential. The amplifier 33 comprises three stages so that the polarity of these voltages is reversed at the output of the amplifier so that negative voltage pulses applied to the oscillator keyer 35 produce marking tones and positive pulses produce spacing tones. These pulses are of square wave form by reason of the neon tube coupling the plate circuit of the amplifier 33 to the grid of the oscillator keyer 35. The oscillator 36 is a typical Hartley oscillator the circuitry of which includes means for frequency adjustment, both as to the marking and spacing signals. Such audio shift signals from the oscillator are passed through the adjustable 'gain buffer amplifier 37 and applied to the transmitting filter 38 tuned to pass only one carrier frequency with the corresponding Mark and Space frequencies and to attenuate adjacent carrier channels by more than 40 db. It will be noted that the output of the transmitting filter is applied to the equipment terminals 10 in parallel with the input side of the receiving filter 18. The filter 18 will reject the signals passed by the filter 38 and such signals are, therefore, transmitted to the remote equipment connected to the terminals 10 by a two-wire line. When the selector magnet loop is keyed by tube 11 the voltage input from the loop circuit to amplifier 33 will vary in amplitude for Mark and Space signals but such voltage will always be positive and of suicient magnitude to exceed the negative grid bias voltage on amplifier 33, thereby causing a steady Mark tone to be generated by the transmitting section. In order to send Mark and Space signals the printer loop must be keyed by the individual printer contacts 16. For this purpose the loop circuit of the printer must be closed by the keying tube 11 either by a continuous marking signal applied from the remote equipment or by the voltage from the Mark-Hold circuit. ln the former case the remote equipment must have its transmitting channel operatively connected tothe transmission line.

Reference is now made to Figure 2 which is a fragmentary diagram showing the circuitry of only such portions of the Figure 1 equipment as are believed necessary for complete understanding of the invention. Here, the various components are separated by dotted lines in accordance with function, specifically, Transmitter D.C. amplifier, Receiver D.C. amplier, Mark-Hold circuit, Keying circuit and Teleprinter.

The function of the Receiver D.C. amplifier is to amplify the D.C. voltages from the discriminator for application in the form of square waves to the telegraph loop keying circuit. Two stages of amplification are included whereby the polarity of the voltages from the discriminator will appear unchanged in the output of the amplifier; positive square wave voltage pulses for a Mark signal and negative, square wave voltage pulses for a Space signal. Signal voltages from the discriminator circuit are applied through a 100,000 ohm resistor 40 to the grid of the first amplifier stage 22, which stage may be one half of a type 5814 dual triode. A xed, negative bias voltage of 4.5 volts is also applied to such grid from a -32 volt regullated supply and a voltage divider consisting of the resistors 41 and 42. This stage operates as a high gain voltage amplifier with a 220,000 ohm plate load resistor 44. Plate voltage for the anodes of the dual triode tube (stages22 and 23) is provided from a 150 volt regulated plate supply. The grid 45 of the stage 23 has a fixed negative bias of approximately -15 volts provided by the -32 volt supply and a voltage divider consisting of the resistor 46, and the resistors 47, 48 in the mark-hold circuit. Coupling between the plate 50 and the grid 45 is by means of a type NE17 neon tube 24. A negative spacing signal, from the discriminator circuit or a fixed negative bias applied to the grid of the tube 22 is sufiicient to cut-off the iiow of plate current. Under such condition the plate voltage of the tube will be of a level high enough to cause the neon tube 24 to ignite. Resistors 44 and 46 will act as a voltage divider between the plate supply voltage and the negative bias supply so that the grid 45, of tube section 23, is driven by a positive voltage which will, of course, produce zero voltage at grid 45 due to the iiow of grid current. Under this condition maximum plate current will ow. On the other hand, positive marking signals from the discriminator circuit will exceed the fixed negative bias applied to the grid of tube section 22 causing a maximum plate current ilow thereby dropping the plate voltage to a level which will cause neon tube 24 to be extinguished. This will open circuit the voltage divider comprised of the resistors 44, 46 and the neon tube 24 so that the grid 45, of tube section 23, will receive the full negative bias voltage which is sufficient in magnitude to cause plate current cut-off in tube section 23. The resistor 49 serves as a plate load and also is part of a voltage divider comprising the neon tube 25, also type NE17, and the resistor 50. This voltage divider is also connected between the plate voltage supply and the -32 volt negative supply. The neon tube 25 couples the plate of tube section 23 to the control grid of the keying tube 1I. The operation of the amplifier tube section 23 is similar to that of the preceding section 22 whereby the normal phase reversal will result in a f 8 positive voltage being applied to the keying tube grid when a Mark signal is received. At the input to the receiver D.C. amplifier the transition from Mark to Space signals, and vice versa, is of sloping characteristic. However, the neon tubes 24 and 25 have discrete firing voltage points which will result in square Wave pulses at the grid 17 of the tube 11. These square wave pulses will also be of constant amplitude due to the voltage regulating characteristics of the type NE17 tubes and the voltage regulation employed in both the plate voltage supply and in the negative bias voltage supply. With neon tube coupling of this type and in view of the magnitude of the voltages employed in the amplifier, variations in the characteristics of the vacuum tube will introduce a minimum of telegraph keying bias. When no signals are received the fixed bias at the input of the D.C. amplifier will cause a steady spacing voltage of -32 volts to be applied to the grid of the keying tube.

The mark-hold circuit, essentially, is a two stage amplifier similar in operation to the receiving amplifier just described. Such mark-hold circuit is driven by the first stage 22 of the receiver D.C. amplifier and its output is applied to the control grid of the keying tube 11. In such arrangement three stages of amplication are provided between the discriminator output and the keying tube, in contrast to two stages of amplification for the signals passing through the receiver D.C. amplifier. Therefore, the polarity of the mark-hold circuit output will be opposite to that of the receiver D.C. amplifier output. It will be seen that the output of the receiver D.C. amplifier is connected in parallel with the output of the mark-hold circuit through the type NE17 neon tube 25 and the neon tube 29 (the latter being a type NESl), in conjunction with the resistor 50 from the 32 volt supply and this combination is connected to the grid of tube 11. The plate of amplifier tube section 22 is coupled by neon tube 24 to the grid 45 of the ampliiier section 23 and also to the grid 51 of the markhold tube section 26 through a voltage divider consisting of the resistors 47 and 48. The -15 volt fixed bias applied to the grid 45 is dropped to about one half this value by the divider for application to the grid 51 in the mark-hold circuit. This arrangement insures that signals must be of sufficient level to dependably operate the receiver D.C. amplifier before actuating the markhold circuit. The two amplifier sections in the markhold circuit comprise a single, dual triode vacuum tube such as a type 5814. The plate of the tube section 26 is coupled to the grid 52 of the tube section by a type NE17 neon tube 30 and a 100,000 ohm grid resistor 53. A fixed bias of approximately -7 volts is also applied to the grid 52 through the resistor 54 from a voltage divider consisting of the resistors 55 and 56 across the -32 volt regulated supply. A capacitor 57 is also connected to the grid 52.

During the time of one marking pulse from the discriminator circuit the neon tube 24 is extinguished as described hereinabove. The negative fixed bias applied to the grid 51 will prevent plate current ow through the load resistor 59 and the voltage at the plate of the tube section 26 will be high enough to cause the neon tube 36 to ignite. The combination of resistors 59 and 53 and the neon tube 30 in series with the time delay network consisting of the 'resistors 54, 55, 56 and capacitor 57, acts as a voltage divider between the l5() volt supply and the -32 volt supply to drive the grid 52 to zero bias. Plate current flow through the load resistor 6i will drop the plate voltage to a level Where the neon tube 29 will be extinguished and the mark-hold circuit will in effect be removed from the keying circuit. However, when a negative spacing signal from the discriminator causes neon tube 24 to be ignited the grid 51, of tube section 26,

is driven to zero and the resulting drop in plate voltage' the grid 52, ofthe mark-hold tube section 27, cannot change instantly due to the time constant of the .R-C network in the grid circuit. However, if the spacing signal time exceeds that which is normally encountered in the telegraph printed character, lor if a continuous spacing voltage is applied to the receiver amplilier, then the neon tube 2,4 remains ignited and the neon tube 30 remains extinguished. The fixed bias at the input of the receiver D.C. amplifier also simulates a continuous spacing signal when no telegraph signals are applied to the equipment. Under these conditions the capacitor 57 will charge up to the -7 Volt bias thereby cutting olf plate current flow in tube section 27 and permitting the plate voltage to lire the neon tube 29. The series combination of resistor 61, neon tube 29 and resistor 50 then acts as a Voltage divider to apply a continuous positive voltage to the grid of the keying tube 11. Neon tube 25, in the receiver amplifier is extinguished during this time. The R-C circuit inthe -grid circuit of tube section 27 isk so arranged that the negative spacing bias voltage on the capacitor 5.7 vis discharged almost instantaneously when the neon tube 36) is ignited by the iirst received marking signal, thus extinguishing neon tube 29 and allowing the keying circuit to be driven by the receiver amplier. One marking pulse per bod (code character time), which is the minimum in any bod, is suicient to prevent neon tube29 from tiring and holding the-keying tube grid 17 at Zero volts.

Since the neon tube 29 remains ignited when no signals are received such tube is located in the front panel of the equipment and used to indicate signal failure. In those installations where the use of the mark-hold circuit is not required the switch 28 is used to disconnect the markhold circuit from the loop keying circuit. The neon tube 29 is then grounded on one side but still retains the feature of glowing during the time no signals are received.

The printer keying circuit is designed for neutral operation only and employs a keying tube 11, which may be a type 6W6, with its cathode grounded. Essentially, the tube 11 acts as an on-oli switch in series with the telegraph printer and the supply battery 14. yAs is now apparent, keying is accomplished by positive marking pulses from the receiver D.C. amplifier which, when applied to the grid v17 through the grid current limiting resistor 62, product saturation plate current in the loop and negative spacing pulses which bias the grid for plate current cut-off in the loop. The 100 ohm resistor 63 is used as a screen voltage dropping resistor so that the plate 13, when conducting, will always be at a higher voltage level than the screen.

In order to use the printer for transmitting and also to print local copy the loop circuit must be closed either by a continuous marking voltage applied to the grid 17 from the receiver amplifier or from the mark-hold circuit. Such requirement makes it possible to have the received signal break the local copy by a series of negative spacing voltages applied to the grid 17. This feature is of practical value in the event the receiving station wishes to break into the transmitting cycle under conditions of urgency.

The resistorsv65, 66 form a high impedance shunt across the printer selector magnet and the battery 14. Direct current voltage pulses are taken from this point in respect to ground and applied to the transmitting D.C. amplifier. When the printer loop is keyed by the tube 11 these pulses are always positive and equal in magnitude to the voltage of the battery 14 during spacing signals. During marking signals such voltage pulses have a ymagnitude equal to the battery voltage minus the voltage drop across the selector magnet 15. When applied to the transmitter section these pulses produce a steady marking signal by the audio frequency shift oscillator. `On the other hand, when the printer loop is keyed by the contacts 16 of the printer the position of the battery 14 in the loop is such that a positive pulse is applied to the transmitting section when the printer contacts are closed and zero voltage is applied to the transmitting section when the printer `contacts vare open, causing the oscillator to produce marking and spacing tones, respectively.

f The transmitter D.-C. amplifier consists of three stages 70, 71 and72 with regulated plate and grid voltage supplies and designed to drive the frequency-shift oscillator section with an output voltage of square wave form. The input signals tothe amplifier section 70 are the loop voltages in respect to ground appearing across the divider consisting of the resistors 65, 66. A positive voltage occurs during marking current flow in the loop and zero voltage during spacing, or no current ow. The polarity of these voltages is reversed at the output of the third amplifier stage 72. Negative voltage pulses applied to the frequency-shift oscillator keyer produce marking tones andpositive pulses produce spacing tones. An adjustable wavefshaping circuit incorporated in the final two amplifier stages provides means for `controlling the relative length of the square waves which drive the oscillator. This permits adjustment of the telegraphbias in the frequency shift carrier -signal output of the transmitting section.

As shown in the drawing, the lirst stage of the D.C. amplifier may consist of 4one half of a 'dual triode tube, such as a type 5814, having a plate load resistor 75. The resistor 76 serves as a grid current limiting resistor and the resistors 77 and 78 couple both the fixed negative bias and the signal voltages from the printer loop to the grid 79. In this capacity these resistors act as a current limiting device and also .as a voltage divider, in conjunction with resistors 65, 66 for the bias voltage andalso the signal voltage from the loop circuit. The plate of the stage 70 is coupled to the grid of the second stage 71 by a resistor 79', the neon tube 80 and the grid limiting resistor 81. A xed negative bias is applied to the grid 82 through the resistor 83. The plate circuit of the stage 71 includes the resistor 84 and is coupled to the grid 85 of the stage 72 by the neon tube 86 and a grid current limiting resistor 87. A potentiometer 88, in series with the resistor 89 provides the means for adjusting the length of the marking and spacing square waves. Each of the neon tubes and 86 are type NE17.

The transmitter D.C. amplifier functions as follows. When a positive voltage pulse is applied to the grid 79 the plate current ow through resistor 75 drops the voltage at the plate to a level below the firing voltage of the neon tube 80. The grid .82, of the stagel 71 then receives the full negative bias voltage applied through the resistor 83. A transition from marking to spacing in the loop circuit removes the positive voltage from the grid 79 and the fixed negative bias immediately cuts off the How of plate current in that tube. The rising plate voltage ignites neon tube 80 whereby the voltage on the grid 8 2 changes from .-32 volts to zero. However, such voltage change is not instantaneous due to the time required for the capacitor 90 to reach zero charge from the -32 volt level. A transition from spacing to marking in the printer loop will cause neon tube S0 to extinguish and the grid S2 will again be biased to cut-off almost immediately as the capacitor charges to -32 volts. This is due to the fact that the critical point of keying voltage at the grid S2 is in a relatively small region near zero as well as the fact that the voltage-time curve of the capacitor has a steeper slope at the start of its charge than it does when approaching maximum charging voltage. The net result of the keying effect on the grid 82 is to lengthen the marking time in respect to the spacing time and to cause the neon tube 86 to ignite for a marking signal and to extinguish for a spacing signal. Instantaneous changes of voltage applied to the grid 85, of the last stage 72, is prevented by the capacitor 91. Spacing and marking voltages will be -32 volts and zero, respectively. The critical keying voltage on the grid is in a narrow range near zero volts, which in this case occurs von the least sloping part of the charging curve of capacitor 91 at the transition of spacing to marking conditions (-32 volts to zero volts). This has the effect of lengthening the spacing signal time. By adjusting the potentiometer 88 the effectiveness of the capacitor 91 can be changed so that the duration of the spacing voltage is greater or less than the marking voltage on the grid 85. The plate of the amplifier section 72 is coupled to the circuit of the frequency shift oscillator keyer by the type NE17 neon tube 34. The output is of square wave form with a negative voltage applied to the oscillator keyer circuit through the resistor 93 when the low plate voltage of stage 72 extinguishes neon tube 34 and a positive voltage being applied when the plate voltage rises to ignite the neon tube. Regulation of the plate and grid supply voltages of the transmitter D.C. amplifier, in addition to the voltage regulation characteristics of the type NE17 neon tubes insures an output wave of constant voltage magnitude and minimizes any change in telegraph bias due to variations in the vacuum tube characteristics.

From the above description it will be apparent that I have provided a relatively simple teleprinter arrangement which allows one printer to be used for both transmitting and receiving. Under receiving conditions the one equipment converts the frequency shifted carrier signals of the transmitting equipment into corresponding positive and negative signal pulses. The negative (space signal) pulses cut-off the keying tube of the receiving equipment whereas positive (mark signal) pulses cause the keying tube to conduct thereby closing the circuit through the selector magnet in the printer loop. The received signals do not operate the D.-C. amplifier in the transmitter channel of the receiving equipment since the first stage of such amplifier has its grid overdriven by a positive voltage. The latter condition causes the transmitting portion of the receiving equipment to send a steady marking signal over the lines connecting the two equipments. Only when a spacing signal is received is the printer loop circuit of the receiving equipment opened by the keying tube.

When the equipment is operated for transmitting information the operation of the printer keyboard breaks the printer loop circuit, the latter being normally closed by the keying tube either by reason of the steady Mark signal sent out by the remote equipment or by operation of the mark-hold circuit. Such opening of the printer loop circuit by keyboard operation removes the positive biasing voltage on the grid of the input stage of the amplifier in the transmitting channel. The depression of a particular key results in an opening and closing of the printer loop circuit in accordance with the pulses corresponding to the character represented by the depressed key. These pulses (Mark and Space signals) produce corresponding positive and negative voltage conditions on the grid of the transmitting D.-C. amplifier and result in corresponding shifts in the frequency of the associated oscillator. Such frequency-shifted wave pulses are passed over the transmission lines through the filter in the transmitting channel of the equipment.

Each teleprinter equipment is provided with two sets of filters connected in parallel to the transmission lines, one such set of filters being tuned to pass only the range of frequency shifted wave pulses of the carrier frequency of the transmitting oscillator and the other set of filters being tuned to pass only the range of frequency shifted wave pulses of the carrier frequency of the associated, remote equipment. The carrier frequencies of a pair of connected equipments may differ by any siutable amount as, for example, 180 and 360 cycles per second with a frequency shift of, say, i4() cycles for the Mark and Space signals. The different frequency sending tones of each equipment are transmitted in each direction over the transmission lines with the receiving filter of one equipment rejecting its own tones and accepting only the tones originating in the other equipment.

The novel mark-hold circuit requires no relays, opens quickly upon receipt of a transmitted Mark signal and holds the printer loop circuit closed when the signal fails thereby preventing the printer from running Wild as would be the case under a no-signal condition. Also, the markhold circuit permits the local operator to transmit messages without first receiving a signal from the remote equipment.

While I have described my invention with specific reference to a teleprinter system wherein two, remotelypositioned printers are connected together by a pair of wires or a radio link, for both transmitting and receiving functions, the `invention is not restricted to such specific application. The novel circuit arrangements including the mutually-exclusive receiving and transmitting filters, the control of the printer loop circuit by a voltage-biased control member such as by the keying tube, the markhold circuit and the steady transmission of a marking tone by each equipment except during keyboard operation, may be used in conventional half-duplex as well as full duplex teleprinter operation. Also, the arrangement is adaptable for keying from the loop circuit of a standard AN/FCC-3 receiver. In such receiver the marking and spacing pulses from the loop are both positive voltages which differ in magnitude. By placing a voltage divider across the input of the transmitter D.-C. amplifier input stage the grid of the amplifier may be driven to zero bias by the marking voltage pulse from the AN/FCC-S receiver, and during a spacing voltage pulse said grid is driven by a negative voltage thereby keying the frequencyshift oscillator.

Having now described my invention in detail in accordance with the requirements of the patent statutes those skilled in this art will have no difficulty making changes and modifications in the disclosed circuitry to meet specific requirements. It will be understood that such changes and modifications may be made without departing from the scope and spirit of the invention as set forth in the following claims.

I claim:

1. A keying circuit for a teleprinter of the class including an electrically energized selector magnet and a fingeroperable keyboard, said keying circuit comprising a D.-C. voltage source; a normally-closed keyboard contact adapted to be opened upon actuation of any key of the keyboard; a control member having an input circuit and an output circuit which output circuit has a relatively high impedance in the absence of a voltage applied to the input circuit; circuit elements connecting the selector magnet, the normally-closed keyboard contact and the output circuit of the control member in series circiut relation across the said D.C. voltage source, the impedance of said output circuit being of a sufficiently high value to' prevent energization of the selector magnet in the absence of a voltage applied to the input circuit of the control member; and a resistor connected in parallel with the output circuit of the control member; the recited arrangement being such that a voltage drop exists across the said resistor except when the normally-closed keyboard contact is Opened in response to actuation of key on the keyboard.

2. In a communication system of the class including a teleprinter having a selector magnet energized in correspondence with voltage pulses produced by an electronic signal receiver and a keyboard operable to produce voltage pulses in an electronic signal transmitter; an arrangement automatically conditioning the teleprinter for both receiving and transmitting operation, said arrangement comprising control means having an input circuit and an output circuit, said output circuit being connected in series with a source of D.C. voltage, the selector magnet and a normally-closed keyboard contact, and said output circuit having an impedance of sufficiently high value to prevent actuation of the selector magnet in the absence of a voltage applied to the said input circuit; circuit elements applying the voltage pulses of the signal receiver to the input circuit of the said control means; and circuit elements applying a voltage developed in the output circuit of said control means to the input circuit of the signal transmitter.

3. The invention as recited in claim 2, wherein the input circuit of the signal transmitter includes an electron tube having a control grid normally biased to prevent tube conduction, and the voltage developed in the output circuit of the control means is developed across a resistor and has a positive magnitude exceeding the normal bias on the said control grid.

4. The invention as recited in claim 3, including means applying a continuous voltage to the input circuit of said control means in the absence of Voltage pulses produced by the signal receiver for a predetermined period of time, and means automatically removing the continuous voltage applied to the input of the control means when voltage pulses are produced by the signal receiver.

5. In a communication system of the class including a teleprinter having a selector magnet energized in accordance with voltage pulses produced by an electronic signal receiver and a keyboard operable to produce voltage pulses in an electronic signal transmitter; an arrangement automatically conditioning the teleprinter for receiving and transmitting purposes, said arrangement comprising a control member having an input circuit and an output circuit, said control member being of a type wherein the said output circuit has a relatively high impedance in the absence of a voltage applied to said input circuit; circuit elements connecting the input circuit of the control member to the output circuit of said signal receiver; circuit elements connecting the output circuit of said control member in series with a source of D.C. Voltage, a normally-closed keyboard contact and the selector magnet; a resistor connected in shunt across the output circuit of said control member; and circuit elements applying a voltage developed across said resistor to the input circuit of said signal transmitter.

6. The invention as recited in claim 5, wherein the voltage developed across said resistor is of a sense and magnitude suicient to cause a continuous signal to be transmitted by the signal transmitter except when the normally-closed keyboard contact is open.

7. The invention as recited in claim 6, including means applying a continuous voltage to the input circuit of the control member, and means removing said continuous voltage from the input circuit of the control member upon the production of a voltage pulse in the signal receiver.

8. In a communication system of the class including a teleprinter having a selector magnet energized in correspondence with positive voltage pulses produced by an electronic signal receiver and a keyboard operable to produce voltage pulses in an electronic signal receiver; an arrangement automatically conditioning the teleprinter for receiving and transmitting purposes which arrangement comprises an electron keying tube having a cathode, an anode and a control electrode; a source of D.C.

voltage; circuit elements connecting theselector magnet, a normally-closed contact on the keyboard and the D.C. voltage source in series relation across the anode and cathode of the keying tube; a resistor connected between the anode of the keying tube and the D.C. voltage source; circuit elements connecting the output of the signal receiver across the grid and cathode of the keying tube; and circuit elements connecting the said resistor to the input circuit of the signal transmitter.

9. The invention as recited in claim 8, including means applying a continuous positive voltage across the grid and cathode of the keying tube in the absence of voltage pulses in the signal receiver for a predetermined time period, and means etective upon production of voltage pulses in the signal receiver to remove the said continuous voltage from the keying tube.

10. In a communication system of the class includingA a teleprinter having a selector magnet energized in correspondence with positive voltage pulses produced in an electronic signal receiver and a linger-operable keyboard operable to produce voltage pulses in an electronic signal transmitter: the combination of an input circuit to the signal transmitter which input circuit includes a tube having a grid normally biased at a negative potential; an electron keying tube having an input circuit and an output circuit, said output circuit having a relatively high impedance in the absence of a positive voltage applied to the input circuit; circuit elements applying the output of the said signal receiver to the input circuit of the keying tube; a source of D.C. voltage; circuit elements connecting the output circuit of the keying tube in series relation with the D.C. voltage source, the selector magnet and a normally-closed keyboard contact; a resistor connected in parallel with the output circuit of the keying tube so that a voltage drop exists across said re sistor at all times except when the normally-closed keyboard contact is open; and circuit elements applying at least a portion of the Voltage drop across said resistor to the grid of the said tube of the signal transmitter and in a sense opposite to the said negative potential, the magnitude of such portion of the voltage exceeding the said negative potential irrespective of whether or not a positive voltage is applied to the input circuit of the keying tube.

11. The invention as recited in claim l0, including means applying a continuous positive voltage to the input circuit of the keying tube in the absence of positive voltage pulses produced by the signal receiver for a predetermined time interval; and means automatically removing the said continuous positive voltage upon the receipt of voltage pulses by the signal receiver.

References Cited in the ile of this patent UNITED STATES PATENTS Hansell Jan. 24, 1950 

